The invention relates to a glass optical fiber having a primary coating of UV-cured acrylate resin and a secondary coating of a thermoplastic resin. The secondary coating is in intimate contact with the primary coating.
In manufacturing optical fibers by drawing from a preform, the drawn fiber is immediately provided with a primary coating. The primary coating has a thickness of approximately 60 microns and serves to maintain the integrity of the glass surface of the fiber. The primary coating protects the glass surface from the atmosphere and prevents the formation of scratches, in the glass surface. For example, the fiber could be scratched when it is wound on a reel or when it is provided with a secondary coating.
During cabling, an optical fiber may be exposed to comparatively large forces. The primary coating in these circumstances provides insufficient protection from mechanical damage to the optical fiber.
It is therefore common practice to give the primary-coated optical fiber a secondary coating. The secondary coating provides protection during cabling, and in the cable against mechanical load. The primary and secondary coatings usually consist of a synthetic resin.
The materials from which the optical fiber and the primary and secondary coatings are constructed have mutually different coefficients of thermal expansion. For silica (SiO.sub.2), of which the actual fiber mainly consists, the coefficient of thermal expansion is 5.times.10.sup.-7 /.degree.C. For synthetic resins, the coefficient of thermal expansion is between approximately 0.5 and 2.times.10.sup.-4 /.degree.C.
When the dual-coated optical fiber is exposed to a decreasing temperature, the comparatively thick secondary coating in particular exerts a compressive force on the optical fiber. The compressive force increases as the temperature decreases. The compressive force adversely influences the optical properties of the optical fiber.
For this reason it has already been suggested that the primary coating should consist of a layer having a buffering effect and remaining soft when the temperature drops. Such a buffering layer prevents mechanical damage from being transmitted from or by the secondary coating to the optical fiber. (See G.B. Patent Application No. 2,065,324A). However, a disadvantage of this construction is that the optical fiber will start buckling within the secondary coating if the secondary coating shrinks considerably in the longitudinal direction. If this phenomenon occurs, the attenuation due to microbending of optical fiber increases considerably.
It has also been suggested to provide the primary-coated optical fiber with a loose secondary coating in the form of a synthetic resin tube. In this case the space between the primary and secondary coatings should be filled with a gel or with air so as to prevent mechanical load if any from being transferred to the optical fiber.
However, when the secondary coating shrinks in the longitudinal direction, this loose coating does not prevent the optical fiber from buckling. The inside of the synthetic resin tube is hardly ever entirely smooth either. In this construction also, all these effects may result in microbending of the optical fiber, when temperature variations or vibrations (for example due to road traffic) occur or when other forces are exerted on the cable (upon winding, laying, and the like). (Microbends are bends in the optical fiber which extend over a few microns to a few millimeters.)
As long as microbending remains in existence, large radiation losses and mode coupling may be the result. As a consequence, the transmission properties of the optical fiber are adversely influenced. The presence of a layer of a lubricant between the primary and the secondary coating provides some but no essential improvement in this respect.
It has also been suggested to make the secondary coating with a synthetic resins having elastic properties which vary only slightly in a given temperature range. A real solution to the problem may not be expected from this suggestion either. The increase of the attenuation still is 1 to 2 dB per km with a drop of the temperature to -50.degree. C. (see in this connection German Auslegeschrift No. 2,723,587 , corresponding to U.S. Pat. 4,114,981).
A glass optical fiber having a primary coating of a UV cured acrylate resin and a secondary coating of a UV cured acrylate resin or a thermoplastic copolyester in intimate contact with each other is known from an artical entitled "UV Cured Resin Coating For Optical Fiber/Cable" by G. L. Schief, et al (Journal of Radiation Curing, April 1982, pages 11-13). Since there is no indication otherwise, it must be assumed that the properties of the fiber have been determined at a standard temperature of approximately 25.degree. C.
During investigations which led to the present invention, Applicants found that in the Schief, et al construction a large increase in the attenuation of the light signal can occur with temperature fluctuations between -50.degree. and +80.degree. C. In certain circumstances this may be a few dB/km. In practice this is not permissible.
G.B. Patent Application No. 2,065,324A describes a construction having a first buffering layer, a second smooth layer, and a third protective layer. In this construction the buffering layer remains soft at decreasing temperatures. Applicant found, however, that microbending and/or macrobending at low temperature is not prevented.
It was furthermore suggested in German Pat. No. 3,002,363 (corresponding to U.S. Pat. No. 4,334,733) to use a secondary coating consisting of a modified polyamide having a lower modulus of elasticity. The disadvantage of this is that the coated fiber can withstand less forces.